1. Field of the Invention
The present invention relates to an image forming apparatuses such as a multi-function printer (MFP), copier, printer or facsimile machine, and to a method for controlling the same.
2. Description of Related Art
Conventionally, in some image forming apparatuses using toner, such as multi-function printers, copiers, printers, and facsimile machines, there are arranged a photoconductive drum and, opposite it with a gap in between, a developing roller. To the developing roller, a so-called developing bias is applied that has a direct current (DC) and an alternating current (AC) superimposed on each other. As a result, charged toner flies from the developing roller to the photoconductive drum, and thereby an electrostatic latent image is developed. The toner image thus developed is transferred onto and fixed to a sheet, and thereby printing is achieved.
Here, to feed sufficient toner to the photoconductive drum, to obtain desired density in the image formed, and to enhance development efficiency, the peak-to-peak voltage of the AC voltage applied to the developing roller may be increased; however, if it is increased too far, electric discharge occurs in the gap between the photoconductive drum and the developing roller. When electric discharge occurs, due to a potential change on the surface of the photoconductive drum, the static latent image is disturbed, and the quality of the image formed is deteriorated. The photoconductive drum can have a property such that, depending on the direction in which the discharge current flows, a large current may flow through the photoconductive drum. When a large current flows, the photoconductive drum may suffer damage, such as a minute hole (pinhole) developing in it. Accordingly, the peak-to-peak voltage may be increased, but within the range in which no electric discharge occurs.
Thus, there is conventionally known a developing unit provided with an image carrying member and, opposite it at a desired interval in the developing region, a toner carrying member, wherein a developing bias voltage having a DC voltage and an AC voltage superimposed on each other is applied between the toner carrying member and the image carrying member so that toner is fed to the image carrying member to develop an electrostatic latent image, there are provided a leak generating means for varying a leak detection voltage applied between the image carrying member and the toner carrying member and a leak detecting means for detecting leakage, wherein, as the maximum potential difference ΔVmax between the leak detecting voltage and the surface potential of the image carrying member is increased, when the current flowing between the image carrying member and the toner carrying member increases continuously, the leak detecting means recognizes leakage.
Here, as in a case where an electric discharge start voltage is searched, electric discharge to be detected may be minute. When electric discharge is minute, the greater a resistance value of a resistor that converts a current on occurrence of electric discharge into a voltage, the larger a range in which a voltage on occurrence of electric discharge varies. Accordingly, it is possible to detect electric discharge with increased sensitivity. As the resistance value of the resistor is increased, however, when, during printing, there is a change in the potential of the developing roller, such as a rise in the potential due to an external factor, there appears a large change in a feedback voltage fed to a direct-current (DC) application portion that applies a DC voltage to the developing roller. As a result, the DC voltage application portion stops outputting or reduces an output voltage, causing a problem that the output voltage of the DC voltage application portion becomes unstable. When the output voltage of the DC voltage application portion becomes unstable, there arises a problem that may affect the quality of images, such as an error in the density of the images to be formed.
Incidentally, some conventional developing apparatuses have, as a configuration for detecting leakage (electric discharge), a current detector detecting a current flowing on occurrence of electric discharge; a specific configuration of that current detector varies, and may not be one that performs no feedback of a direct current applied to the developing roller. Accordingly, with the conventional developing units, it is impossible to solve the above-described problems.